A concentration control system that is used for this sort of source vaporization system and intended to keep a concentration of a source gas constant is under application by the present applicant. The concentration control system includes, on an outlet line through which mixed gas of carrier gas and the source gas is brought out of a tank: a first valve; a partial pressure measurement sensor that measures a partial pressure of the source gas; and a total pressure measurement sensor that measures a total pressure corresponding to a pressure of the mixed gas, and is provided with: a concentration measurement part that measures the concentration of the source gas in the mixed gas by dividing the measured partial pressure by the total pressure; and a concentration control part that controls a stroke of the first valve such that the measured concentration measured by the concentration measurement part becomes equal to a predetermined concentration setting.
According to the concentration control system configured as above, even if a source inside the tank is not vaporized at a saturated vapor pressure, and a state of the vaporization is varied to vary the partial pressure of the source gas, the total pressure of the mixed gas is controlled by the first valve to meet the variation. Accordingly, regardless of a generation situation of the source gas in the tank, the concentration of the source gas can be kept constant at the concentration setting.